Conventional mechanical circuit breakers have been used for a long time for interruption of fault currents. After having detected a short circuit or an over-load situation, some time (several periods of the electrical line frequency) elapses prior to an opening of the switches mechanically. Subsequently, an arc occurs, which initially has little impact on the current. The current can only be quenched at its natural zero-crossing assuming that the plasma in the region of the contacts of a mechanical circuit breaker is significantly cooled down to avoid re-ignition. As a result, turning off a short circuit will take at least 100 ms (without detection time), i.e., several line periods.
Because of the thermal and electrical stresses inherent in opening and closing of conventional circuit breakers, such breakers have traditionally been very large and expensive devices, requiring expensive maintenance after a number of switching operations. Arcing which occurs across the contacts during interruption of a fault current can damage contact electrodes and restrict nozzles of the mechanical circuit breaker. For this reason conventional circuit breakers require frequent inspection and expensive maintenance. The problem of arcing becomes very acute for breaker applications where high switching frequency is required such as conveyor drives, inching and reverse operations, industrial heaters, test beds etc. The number of high-current short circuit clearances is limited to about 10 to 15 times for contemporary mechanical devices.
The peak current cannot be influenced using these classical mechanical circuit breakers. Therefore, all network components have to withstand the peak current during the switching period. Mechanical circuit breakers also have a maximum short circuit current rating. This current limit forces designers of electric grids to limit the short circuit power of the grids, e.g., by using additional line inductances. However, these measures also reduce the maximum transferable power and the “stiffness” of the grid, leading to an increase of voltage distortions. During the short circuit time, the voltage on the complete grid is significantly reduced. Due to the long turn-off delay of the breaker, sensible loads require UPS support to survive this sag, which is costly and might not be feasible for a complete factory plant.
The latest progresses in power electronics make realistic the replacement of these mechanical type circuit breakers by semiconductors, in order to get very fast systems. Such static circuit breakers based on high power semiconductors potentially offer enormous advantages when compared to conventional solutions, since a solid-state breaker is able to switch in a few microseconds. They also require very little maintenance. Due to the absence of moving parts there is no arcing, contact bounce or erosion. Recently, considerable progress has been made in the development of low power solid-state breakers for AC and DC applications. The main disadvantage of the solid-state breaker is the high thermal losses generated by the continuous load current. Electronic switching devices, such as thyristors, IGBTs and GTOs, always have a voltage drop across their terminals resulting in heating through the I2R loss. The amount of heat depends on the current. As the current increases, this drawback starts to mount and large heat sink becomes a necessity. At very high currents, the electromechanical breaker remains firmly established, with no short-term likelihood that the solid-state breaker replacing it.
Based on experience, it can be concluded that there are basically three requirements that a circuit breaker must meet. First, during its conducting state, it must conduct large currents with minimal power loss. Second, in the event a fault is detected, it should be capable of transitioning itself to its blocking state without self destructing in the process. Finally, it must then, of course, block any current from flowing despite high potentials on its terminals. Mechanical circuit breakers, by their construction, are ideally suited for the first and last of these requirements, but they could fail in the second requirement, due to large circuit inductance, unless sufficient design tolerances are used. Semiconductor switches, on the other hand, because of their small but still finite on-state resistance, are unsuitable for the first requirement, yet can still perform admirably for the other two. It is a distinct possibility therefore that a parallel combination of semiconductor switch and mechanical breaker might well combine the advantages of both and, at the same time, reduce the requirements that either would need if used alone.
The essential idea of this hybrid breaker, which forms prior art, is to detect the fault through normal means and initiate the opening of the mechanical breaker. After a few hundreds of arc volts have been reached the parallel semiconductor switch can be closed. Current transfers to the semiconductor switch and the mechanical breaker opens fully and clears. The semiconductor switch is then opened by an appropriate signal (or lack of signal) on its control electrode and the current is passed to a third parallel device which constitutes a dissipative network for the inductive fault current, leaving the hybrid breaker system open and clear, blocking the full source potential which may be hundreds of kV. The dielectric and mechanical stresses on the mechanical breaker are much reduced in this system since at no time during its opening process does the mechanical breaker ever see much more than the low voltage needed to trigger the semi-conductor device, nor does it at any time see the full fault current (potentially many kA) arcing on its terminals. This hybrid breaker should therefore allow breakers to be built that are more reliable and have higher power ratings and faster response and re-closure times, and which, in addition, have the capability of multiple operations.
Nevertheless, the use of the conventional AC mechanical breaker in combination with a solid state device is challenging due to:                1. Different reaction times (fault detection, interruption times) required for the two components, i.e. the interruption time tint of the conventional AC mechanical breaker is in the scale of m sec<tint<sec meanwhile the interruption time tint of a controllable solid-state device, IGBT, is in the range of μ sec<tint<m sec. Current interruption through the solid-state device can be in the range of a couple of microseconds if the stray inductance of the circuit is very low.        2. Different current rating capabilities, i.e. the conventional AC mechanical breaker can interrupt a fault current of some tens of kA but on the other hand controllable solid-state devices, such as IGBTs, can interrupt currents of only some kA.        3. Arc voltage. The fact that the higher the fault current the higher the arc voltage. In order to be able to commutate the current from the mechanical breaker to the solid-state device an arc voltage which is double as high as the solid-state device voltage drop is required.        4. Commutation time. If the loop inductance is high then high commutation time is required. High commutation time results in a further increase in magnitude of the fault current and therefore the solid-state device is forced to interrupt very high currents.        5. Conduction time of the solid-state device is critical due to:                    a. High-conduction time is required in order to completely commutate the current from the mechanical breaker to the solid-state device.            b. High-conduction time is required when the loop inductance is high            c. High-conduction time is required in order to extinguish the arc voltage of the mechanical breaker, i.e. no current is flowing through the mechanical breaker.                        
High-conduction times result in high conduction losses and as a result overheating of the device which can lead into device failures. As a result, conduction time should be kept as low as possible.
Moreover, the hold-off interval may lead to an extremely high turn-off current, in the range of several kA. This high current would require semiconductors with a high peak current turn-off capability or parallel connection of devices. Since the allowable voltage slope is constant, higher grid voltage will consolidate this drawback, because the hold-off interval must be increased. As an example, for a grid voltage of 30 kV it would be 375 microseconds. For low voltage circuit breakers, this hold-off interval setting also takes into account the overloading conditions, resulting in similar high current flowing requirements through the semiconductors.
As mentioned in the previous section, the standard hybrid circuit breaker suffers from the drawback of long hold-off interval. This drawback could be avoided by either preventing the ignition of an arc or limiting the current peak during the hold-off interval. The present invention primarily aims at preventing the ignition of an arc between the contacts of the mechanical switch during breaking action of the latter.